Do you attach 2 2x10s on one side, or split 1 2x10 on one side and 1 2x20 on the other?

oh'mike

07-05-2012 07:38 AM

What are you building? Sounds like maybe a deck girder?

GCO88

07-05-2012 10:35 AM

Gco88

Building a deck I can either notch out 3" x 10" on one side or notch out 1 1/2" on each side?

Yoyizit

07-05-2012 10:45 AM

Quote:

Originally Posted by GCO88
(Post 958484)

Building a deck I can either notch out 3" x 10" on one side or notch out 1 1/2" on each side?

I'd do 1.5" on each side and since the deck weight is carried by the shoulder you cut into the wood, minimal fasteners would be needed to secure these joists unless there are significant sideways forces.

DexterII

07-05-2012 11:48 AM

deleted

goosebarry

07-05-2012 02:46 PM

1 Attachment(s)

Quote:

Originally Posted by GCO88
(Post 958372)

Do you attach 2 2x10s on one side, or split 1 2x10 on one side and 1 2x20 on the other?

This is at least the second time this has come up since I joined. The answer is maybe. Let me explain.
First split beam designs maybe banned by code or require signoff from a SE, but more importantly it is possible to under size the beams.
For a normal deck the load is assumed to be equally distributed across the surface for sizing purposes. Using this assumption, the length of the joists supported by a beam are linearly proportional to the width of the beam (2 by, 3 by 4 by, double 2 by, etc)If your deck supports point loads or exordinary loads, such as a hot tub, then additional calculations are required. Attachment 53566
The attached drawing shows a simple layout with split beams. The colored boxes represent the area of load for each of the "2 by" beams. If the beam was actually a "double 2 by" then the beam on the right would carry the total load of the blue and green area, while the beam on the left would carry the total load of the red and purple area which in this the load from the red area far exceeds half the total load.

What does this mean in the real world? For most decks people use the beam/joist tables using the maximum joist length from the design (green and red in the attachment). As you can this maximum yields a maximum supported load that is greater than or equal to any load any beam would actually see.

On the otherhand if you calculate and size beams individually, then it is possible that some beams may be undersized when split. For example, you size a double 2 by beam to handle the red and purple loads. Then you decide to split the beam. Remeber the length of a joist (distributed load)that a beam will support is lineraly proportional to the beam width, therefore when spliting a beam each 2 by has to support half the load. In this case the red load far exceeds half the combined load.

Hope that helps. I don't like to simply accept "because the code says", there usually is a logical explanation backed by engineering for what is in the code.

If you are building to the International Residential Code, which is pretty common in the U.S., you are not permitted to split the beam. There is a diagram of the allowed attachment method in the Prescriptive Guide, which clearly shows that you notch the post for the full width of the doubled beam (typically 3 inches), then use 1/2 inch minimum diameter bolts (not lag screws unless allowed by your building inspector, and not carriage bolts unless specifically allowed by your building inspector). Alternatively, you can attach the beam to the top of the post using a Simpson type bracket.

If you are not building to IRC, follow the applicable code requirements. If there is no code in your area, I would follow IRC, it is pretty complete, it is conservative, and it is easy to follow.

Yoyizit

07-05-2012 05:43 PM

I stand corrected.

mae-ling

07-05-2012 06:18 PM

Put them together and use a bracket on top of the post. If you can't find one for a 3" beam then add a filler chunk if you have to.

goosebarry

07-05-2012 06:44 PM

Quote:

Originally Posted by Daniel Holzman
(Post 958722)

If you are building to the International Residential Code, which is pretty common in the U.S., you are not permitted to split the beam. There is a diagram of the allowed attachment method in the Prescriptive Guide, which clearly shows that you notch the post for the full width of the doubled beam (typically 3 inches), then use 1/2 inch minimum diameter bolts (not lag screws unless allowed by your building inspector, and not carriage bolts unless specifically allowed by your building inspector). Alternatively, you can attach the beam to the top of the post using a Simpson type bracket.

If you are not building to IRC, follow the applicable code requirements. If there is no code in your area, I would follow IRC, it is pretty complete, it is conservative, and it is easy to follow.

Daniel,
Do you know when the restriction was added? I remember back in the day it was popular because it was easy to notch and aesthetically pleasing.

Daniel Holzman

07-05-2012 08:37 PM

My town runs under the 2006 IRC, and split beams are not permitted under that code. The restriction may be older, I have not researched any further back.